mand sometimes changes significantly on a timescale of a few minutes.
This chapter discusses how to cope with fluctuations in supply and demand,
without using fossil fuels.

How much do renewables fluctuate?

However much we love renewables, we must not kid ourselves about the
fact that wind does fluctuate.

Critics of wind power say: “Wind power is intermittent and unpredictable,
so it can make no contribution to security of supply; if we create
lots of wind power, we’ll have to maintain lots of fossil-fuel power plant to
replace the wind when it drops.” Headlines such as “Loss of wind causes
Texas power grid emergency” reinforce this view. Supporters of wind energy
play down this problem: “Don’t worry – individual wind farms may
be intermittent, but taken together, the sum of all wind farms in different
locations is much less intermittent.”

Let’s look at real data and try to figure out a balanced viewpoint. Figure
26.2 shows the summed output of the wind fleet of the Republic of
Ireland from April 2006 to April 2007. Clearly wind is intermittent, even if
we add up lots of turbines covering a whole country. The UK is a bit larger
than Ireland, but the same problem holds there too. Between October 2006
and February 2007 there were 17 days when the output from Britain’s 1632
windmills was less than 10% of their capacity. During that period there
were five days when output was less than 5% and one day when it was

Figure 26.2. Total output, in MW, of all wind farms of the Republic of Ireland, from April 2006 to April 2007 (top), and detail from January 2007 to April 2007 (middle), and February 2007 (bottom). Peak electricity demand in Ireland is about 5000 MW. Its wind “capacity” in 2007 is 745 MW, dispersed in about 60 wind farms. Data are provided every 15 minutes by